Aqueous dispersions of a thermoplastic resin of various types are known in the art. Aqueous dispersions have been used in a wide variety of fields since an aqueous dispersion prepared by using water as its dispersion medium is far more advantageous than the dispersions prepared by using an organic solvent for the dispersion medium in view of flammability, working environment, handling convenience, and the like. For example, when an aqueous dispersion is coated and dried on a surface of a substrate such as paper, fiber, wood, metal, or plastic molded article, the resin coating formed will provide the substrate with water resistance, oil resistance, chemical resistance, corrosion resistance and heat sealability.
Conventional aqueous dispersions of a thermoplastic resin have been produced either by a process wherein a polymerizable monomer which is the resin raw material is polymerized by emulsion polymerization in an aqueous medium in the presence of a dispersing agent, or by a process wherein a molten thermoplastic resin and an aqueous medium, and optionally, a dispersing agent are mixed by applying shearing force. The former process is associated with the disadvantage of the limited number of the polymerizable monomers that can be used, and hence, the variety of the aqueous dispersions of the thermoplastic resin that can be produced, is limited. The former process also suffers from complicated control of the polymerization reaction as well as intricate equipment. On the other hand, the latter process is applicable to a wide variety of resins in relatively simple equipment.
One particular application for coatings made from dispersions is in packaging and storage container applications. To be useful, a balance of performance properties such as low heat seal initiation temperature, a high hot tack strength, a broad hot sealing window, good interlayer adhesion, and a high softening point is desirable.
The commercial importance of balanced sealant properties is well understood. That is, low heat seal initiation temperatures are important for improved sealing speeds and reduced energy utilization. A broad sealing window is important for insuring package integrity, sealing equipment flexibility and low package leakage rates.
Good interlayer adhesion is also important for good package integrity as well as good package or container aesthetics. High softening points or temperatures are desired where goods are packaged at elevated temperatures such as in hot-fill applications. Traditionally, when attempting to achieve balanced sealant properties, enhancement of one particular resin property has required some sacrifice with respect to another important property.
For instance, with ethylene alpha-olefin polymers, low heat seal initiation temperatures are typically achieved by increasing the comonomer content of the resin. Conversely, high Vicat softening points and low levels of n-hexane extractives are typically achieved by decreasing the comonomer content of the resin. Accordingly, lowering the heat seal initiation temperature typically results in proportionally reduced Vicat softening temperature and proportionally increased extractable level. U.S. Pat. No. 5,874,139, which is assigned to the assignee of the present invention and is expressly incorporated by reference in its entirety, provides a general discussion of polyolefins in packaging applications.
Several important multilayer packaging and storage structures consist of a polypropylene layer, particularly, a biaxially oriented polypropylene homopolymer (BOPP) base or core layer. Often, BOPP structures utilize polypropylene copolymers and terpolymers as sealant materials (and/or adhesive layers) to insure good interlayer adhesion to the BOPP base layer. While polypropylene copolymers and terpolymers do indeed provide good interlayer adhesion to BOPP base layers as well as good heat seal strength performance, these copolymers and terpolymers sometimes exhibit undesirably high heat seal initiation temperatures.
Other materials have also been used as sealant materials for multilayer packaging and storage structures. However, in general, known sealant materials do not provide the desired overall property balance and/or process flexibility desired by converters and packagers.